This invention relates to a method of making an electrochemical cell, such as a fuel cell or a metal-air cell, with a catalytic electrode including a catalyst for reducing oxygen.
Electrochemical cells can be used to provide electric energy to operate electronic devices. When used to operate electronic devices, these cells are advantageous because they use oxygen from outside the cell as an active material, reducing the size of the electrode consuming the oxygen (e.g., the positive electrode, or cathode) and making more volume within the cell available for the counter electrode (e.g., the negative electrode, or anode) and electrolyte. A hybrid metal-air cell, sometimes referred to as an air-assisted cell, has positive and negative electrodes with active materials that are contained within the cell and an auxiliary catalytic electrode that reduces oxygen from outside the cell as an active material to recharge the primary positive electrode. Examples of fuel cells, metal-air cells and air assisted cells can be found in the following patent publications: U.S. Pat. No. 5,079,106; U.S. Pat. No. 5,229,223; U.S. Pat. No. 5,308,711; U.S. Pat. No. 5,378,562; U.S. Pat. No. 5,567,538; U.S. Pat. No. 6,383,674; U.S. Pat. No. 6,461,761; U.S. Pat. No. 6,602,629; U.S. Pat. No. 6,911,278; U.S. Pat. No. 7,001,689; US 2004/0197641 and US 2009/0214911; and in International Patent Publication No. WO 00/36677.
Fuel cells, metal-air cells and air assisted cells include a catalytic electrode containing a catalyst for reducing oxygen from outside the cell, and a product of the oxygen reduction reaction reacts with the active metal in the counter electrode (e.g., the negative electrode in fuel cells and metal-air cells or the primary positive electrode in air assisted cells). The properties of the catalyst can affect the electrical performance of the cell. Improved cell performance, such as greater discharge capacity or better high rate and high power discharge capability, is desirable.
Manganese oxides are known as oxygen reduction catalysts, and many types of manganese oxides have been used as catalysts in metal-air cells, both alone and in combination with other catalysts such as silver and platinum metal. The manganese oxides can include manganese in a range of valences, either primarily a single valence or multiple valences. A variety of methods of making manganese oxides for use as catalysts are also known. Manganese oxides are desirable as catalysts because they can have good catalytic activity for the reduction of oxygen, they can be relatively inexpensive, and they can be relatively inexpensive to manufacture. However, many of the processes used to make manganese oxides for use in metal-air cells have disadvantages.
JP 51-071,299 A discloses a method of preparing an activated manganese dioxide catalyst from potassium permanganate and an acid solution of manganese nitrate. The method includes a solution precipitation process in which the reactions take place at 35° C. to 90° C. In this process the nitrate cation may participate in the oxidation-reduction reaction to produce nitrogen oxides that are undesirable from a safety and environmental standpoint. Acid, which can present a safety hazard, is also required.
US 2008/0176059 A1 discloses a composite material including a microporous or mesoporous matrix and nanoparticles of metal or metal oxide formed within the pores of the matrix. The process includes radiolytic reduction of a precursor solution with the pores of the matrix. The material includes a solid matrix material that may not be suitable for some cell constructions, the composite material is much more voluminous than just the catalyst, the irradiation required can present safety and environmental concerns, and the process requires heating and a vacuum.
U.S. Pat. No. 6,632,557 B1 discloses a cathode for a metal-air electrochemical cell that includes both manganese dioxide and silver as oxygen reduction catalysts. The catalyst is prepared by a solution precipitation process using silver permanganate as an oxidizing agent and carbon as a reducing agent in water or isopropanol to form the catalyst on the surfaces of the carbon particles. An additional reducing agent such as hydrazine or hydroxylamine can be used. The catalyst is compared to catalysts made from each of manganese nitrate, a combination of silver nitrate and manganese nitrate, and potassium permanganate. The relatively high cost of silver permanganate, difficulty in achieving a uniform mixture and product due to the insolubility of carbon, large volume of liquid needed due to the limited solubility of silver permanganate can be disadvantages.
U.S. Pat. No. 6,780,347 B2 discloses a method of making a catalyst that can be used in a metal-air cell. The catalyst contains manganese oxides, including manganese dioxide, by reacting potassium permanganate and an organic or inorganic reducing agent (e.g., sodium formate, formic acid or formaldehyde) in an aqueous solution to form a sol, and then mixing the sol with a carbon slurry and Teflon dispersion. The reaction temperature can range from 25° C. to 100° C. The presence of impurities and undesirable reaction products in the sol, which is mixed directly with carbon, can be a disadvantage, and the type of manganese oxide catalyst can vary depending on the reaction temperature.
A manganese oxide that is safe, easy and inexpensive to make and that can be readily incorporated into a catalytic electrode for a fuel cell, metal-air cell or air assisted cell that provides excellent cell performance is desirable.